CN102520403B - Improved frequency stepping synthetic aperture radar (SAR) imaging method based on frequency domain frequency spectrum reconstruction - Google Patents
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Abstract
An improved frequency stepping synthetic aperture radar (SAR) imaging method based on frequency domain frequency spectrum reconstruction specifically includes: 1 grouping frequency stepping SAR echo data stepping at carrier frequency n points to obtain n groups of sub-band echo data (including data of m distance lines) with different carrier frequencies; 2 judging whether aliasing can happen on distance frequency of the sub-band echo data performed with two-dimensional focus processing, and performing expanding operation on distance frequency spectrum of the sub-band echo data on yes judgment; 3 performing SAR imaging operation on the sub-band echo data, and removing an overlapped part of the frequency spectrum by windowing the distance frequency spectrum in the distance compression process when sub-band echo band width is larger than stepping frequency interval; 4 performing direction registering on n sub-images; and 5 performing distance frequency spectrum reconstruction on data of a kth distance line taken from each sub-image, repeating for m times and obtaining two-dimensional images with high range resolution. The method removes influence of sub-band data distance frequency spectrum aliasing and ensures distance frequency spectrum reconstruction to be performed smoothly.
Description
Technical Field
The invention relates to an improved frequency stepping SAR imaging method based on frequency domain spectrum reconstruction, and belongs to the technical field of SAR imaging.
Background
Frequency stepping SAR imaging methods based on frequency domain spectrum reconstruction are mainly divided into two types: firstly, distance compression and frequency domain spectrum reconstruction are carried out, and then distance migration correction and azimuth focusing are carried out; and secondly, performing two-dimensional focusing processing and then performing distance frequency domain spectrum reconstruction. For the first method, distance compression operation is required before distance spectrum reconstruction, but the structure cannot be combined with a high-efficiency imaging algorithm, namely a CS algorithm, so that the adaptability of the first method is poorer than that of the second method. However, the second method has a problem: after the two-dimensional focusing process, the range spectrum of the echo data may be distorted, especially to an increased extent in high-resolution SAR systems, which is likely to result in range spectrum aliasing. If such distance spectrum aliasing occurs, the distance spectrum reconstruction cannot be performed. In the prior art, no method for solving the distance spectrum aliasing phenomenon is found.
Disclosure of Invention
The invention aims to provide an improved frequency stepping SAR imaging method based on frequency domain spectrum reconstruction, aiming at the problem of distance spectrum aliasing possibly existing in the conventional frequency stepping SAR imaging method (the second method mentioned in the background technology) which firstly performs two-dimensional focusing treatment and then performs distance frequency domain spectrum reconstruction.
An improved frequency stepping SAR imaging method based on frequency domain spectrum reconstruction includes steps one to five, specifically:
step one, grouping frequency stepping SAR echo data with n-point stepping carrier frequencies according to signal carrier frequencies to obtain n groups of sub-band echo data with different carrier frequencies, wherein n is more than or equal to 2 and n is a positive integer; each group of sub-band echo data comprises m pieces of data of distance line, and m is a positive integer.
Step two, judging whether the distance spectrum of the sub-band echo data in the step one is subjected to aliasing after two-dimensional focusing processing; if the frequency spectrum aliasing occurs, performing expansion operation on the distance frequency spectrum of the sub-band echo data, and then performing operation in the third step; otherwise, directly carrying out the operation of the third step.
The method for judging whether the distance spectrum of the sub-band echo data is subjected to aliasing after two-dimensional focusing processing specifically comprises the following steps: if Δ f ≧ Δ fthresholdAnd judging that the distance spectrum of the sub-band echo data is subjected to aliasing after two-dimensional focusing processing. Wherein Δ f is a step frequency interval; Δ fthresholdFor the purpose of the threshold step interval,
fsis the sampling rate; beta is aAIs the azimuth beam width; f. of0Is the carrier frequency; b isrIs a sub-band echo bandwidth and BrIs more than or equal to delta f. If Δ f < Δ fthresholdAnd judging that the distance spectrum of the sub-band echo data does not alias after the two-dimensional focusing processing is carried out on the sub-band echo data.
The specific method for expanding the distance spectrum of the sub-band echo data comprises the following steps: converting sub-band echo data into a distance frequency domain through Fourier transform, and expanding the range of the distance frequency domain to B through zero filling at two ends of the distance frequency spectrumr_extendHz(ii) a Wherein, alpha is an oversampling coefficient and is more than 1 and less than or equal to 1.3; namely: if the original point number in the distance direction is marked with the symbol NrIndicating that the number of zero-filling points at both ends of the spectrum is denoted by the symbol Nr_zerosIt is shown that,
indicating rounding up.
And step three, respectively carrying out SAR imaging operation on the n groups of sub-band echo data obtained in the step two to obtain n sub-images. When B is presentrWhen the frequency is greater than Δ f, the adjacent sub-band spectrums have an overlapping phenomenon, and in order to eliminate the influence of the overlapping of the sub-band spectrums on the spectrum reconstruction, the overlapping part of the spectrums needs to be removed by windowing the distance spectrums in the distance compression processing.
The SAR imaging operation steps include steps 3.1 to 3.3, specifically:
step 3.1: performing distance compression operation on the sub-band echo data to obtain sub-band data subjected to distance compression operation; when B is presentrWhen the frequency spectrum of the adjacent sub-bands is larger than deltaf, the overlapping phenomenon of the frequency spectrums of the adjacent sub-bands occurs, and in order to eliminate the influence of the overlapping of the frequency spectrums of the sub-bands on the frequency spectrum reconstruction, the overlapping part of the frequency spectrums is removed through distance frequency spectrum windowing in the distance compression processing.
Step 3.2: and performing range migration correction on the sub-band data after the range compression operation to obtain the sub-band data after the range migration correction.
Step 3.3: and performing azimuth compression operation on the sub-band data after the range migration correction to obtain sub-images.
And step four, carrying out azimuth registration on the n sub-images obtained in the step three to obtain n sub-images after azimuth registration.
The specific method for carrying out azimuth registration on the n sub-images comprises the following steps: selecting the ith sub-image from the n sub-images as a reference image, wherein i is more than or equal to 1 and less than or equal to n and i is a positive integer, and translating the time shift property of other sub-images through Fourier transform in azimuthAnd each pixel is more than or equal to 1 and less than or equal to n, j is a positive integer and j is not equal to i.
Step five, taking out kth distance offline data from each sub-image obtained in the step four to obtain n pieces of distance offline data, and performing distance spectrum reconstruction on the n pieces of distance offline data, wherein k is more than or equal to 1 and less than or equal to m, and k is a positive integer; and repeating the operation m times to obtain a two-dimensional image with high distance resolution.
The specific method for reconstructing the distance spectrum of the n pieces of distance offline data comprises the steps from 5.1 to 5.4, and specifically comprises the following steps:
step 5.1: and respectively transforming the n pieces of distance off-line data to a distance frequency domain through Fourier transform to obtain n pieces of distance frequency spectrums.
Step 5.2: and (4) carrying out zero filling operation on the n distance frequency spectrums obtained in the step (5.1) to obtain the distance frequency spectrums after the n zero filling operations.
The specific method for zero filling operation on the n distance frequency spectrums comprises the following steps:
if the sub-band echo data corresponding to the n distance spectrums are subjected to distance spectrum expansion in the step two, the zero padding positions of the n distance spectrums are obtained according to the formula (1):
wherein fp isiRepresents the ith webThe zero padding position of the distance spectrum of the kth piece of the subimage from the offline data.
If the sub-band echo data corresponding to the n distance spectrums are not subjected to distance spectrum expansion in the step two, the zero padding positions of the n distance spectrums are obtained according to a formula (2):
step 5.3: frequency shifting the n distance frequency spectrums after zero filling operation obtained in the step 5.2 to obtain n distance frequency spectrums after frequency shifting; symbol δ f for frequency shift amount of k-th line distance off-line data of ith sub-imageiIt is shown that,
step 5.4: and (4) overlapping the n distance frequency spectrums after frequency shift obtained in the step (5.3), and performing inverse Fourier transform operation to complete distance frequency spectrum reconstruction to obtain a high-resolution distance line.
Advantageous effects
Compared with the existing frequency stepping SAR imaging method based on frequency domain spectrum reconstruction, the improved frequency stepping SAR imaging method based on frequency domain spectrum reconstruction provided by the invention has the advantages that the influence of sub-band data distance spectrum aliasing is eliminated, and the smooth operation of distance spectrum reconstruction can be ensured.
Drawings
Fig. 1 is a schematic flowchart of an improved frequency-stepped SAR imaging method based on frequency-domain spectral reconstruction according to an embodiment of the present invention;
FIG. 2 is a two-dimensional spectral plot of point objects in a two-dimensional image with high range resolution in an embodiment of the present invention;
FIG. 3 is a contour plot of a point target in a two-dimensional image with high range resolution in an embodiment of the present invention;
FIG. 4 is a plot of distance to normalized amplitude for point targets in a two-dimensional image with high range resolution in an embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and the specific embodiments.
An improved frequency-stepping SAR imaging method based on frequency-domain spectral reconstruction includes, as shown in fig. 1, steps one to five, specifically:
simulating frequency stepping SAR echo data of a point target, wherein simulation parameters are shown in a table 1; grouping the frequency stepping SAR echo data with 4-point stepping carrier frequency according to the signal carrier frequency to obtain 4 groups of sub-band echo data with different carrier frequencies; each set of sub-band echo data includes 3500 pieces of data of the distance line.
TABLE 1 simulation parameters Table
Step two, judging whether the distance spectrum of the sub-band echo data in the step one is subjected to aliasing after two-dimensional focusing processing; if the frequency spectrum aliasing occurs, performing expansion operation on the distance frequency spectrum of the sub-band echo data, and then performing operation in the third step; otherwise, directly carrying out the operation of the third step.
The method for judging whether the distance spectrum of the sub-band echo data is subjected to aliasing after two-dimensional focusing processing specifically comprises the following steps: if delta f is larger than or equal to 2.9MHz, aliasing of the distance spectrum of the sub-band echo data is judged after two-dimensional focusing processing. And if the delta f is less than 2.9MHz, judging that the distance spectrum of the sub-band echo data does not alias after the two-dimensional focusing processing is carried out on the sub-band echo data. ' Delta f is more than or equal to 2.9MHz under the simulation parameters, so that the distance frequency spectrum of the sub-band echo data is judged to be aliased after the two-dimensional focusing processing is carried out.
The specific method for expanding the distance spectrum of the sub-band echo data comprises the following steps: converting sub-band echo data into a distance frequency domain through Fourier transform, and expanding the range of the distance frequency domain to 18.1Hz at two ends of a distance frequency spectrum through zero padding, wherein alpha is 1.2; since the distance is toward the original 152 points, it is necessary to fill the zero 39 points at both ends of the spectrum.
And step three, respectively carrying out SAR imaging operation on the n groups of sub-band echo data obtained in the step two to obtain 4 sub-images. Due to BrIf the frequency spectrum of the adjacent sub-bands is more than deltaf, overlapping phenomena can occur on the frequency spectrums of the adjacent sub-bands, and in order to eliminate the influence of the overlapping of the frequency spectrums of the sub-bands on the frequency spectrum reconstruction, the overlapped parts of the frequency spectrums need to be removed by windowing the distance frequency spectrums in the distance compression processing.
The SAR imaging operation comprises the following specific steps:
step 3.1: performing distance compression operation on the sub-band echo data to obtain sub-band data subjected to distance compression operation; due to BrIf the frequency spectrum of the adjacent sub-bands is more than deltaf, overlapping phenomena occur on the frequency spectrums of the adjacent sub-bands, and in order to eliminate the influence of the overlapping of the frequency spectrums of the sub-bands on the frequency spectrum reconstruction, the overlapping parts of the frequency spectrums are removed through distance frequency spectrum windowing in the distance compression processing.
Step 3.2: and performing range migration correction on the sub-band data after the range compression operation to obtain the sub-band data after the range migration correction.
Step 3.3: and performing azimuth compression operation on the sub-band data after the range migration correction to obtain sub-images.
And step four, carrying out azimuth registration on the 4 sub-images obtained in the step three to obtain 4 sub-images after azimuth registration.
The specific method for carrying out the orientation registration on the 4 sub-images comprises the following steps: selecting the ith sub-image from the 4 sub-images as a reference image, wherein i is more than or equal to 1 and less than or equal to 4 and is a positive integer, and translating the time shift property of other sub-images through Fourier transform in azimuth
And each pixel is more than or equal to 1 and less than or equal to 4, j is a positive integer and j is not equal to i.
Step five, taking out the kth distance offline data from each sub-image obtained in the step four to obtain 4 pieces of distance offline data, and performing distance spectrum reconstruction on the 4 pieces of distance offline data, wherein k is more than or equal to 1 and less than or equal to 3500, and is a positive integer; 3500 times of operations are repeated to obtain a two-dimensional image with high distance resolution.
The specific method for reconstructing the distance spectrum of the 4 pieces of distance off-line data comprises the following steps:
step 5.1: and respectively transforming the 4 pieces of distance off-line data to a distance frequency domain through Fourier transform to obtain 4 pieces of distance frequency spectrums.
Step 5.2: and (4) carrying out zero filling operation on the 4 distance frequency spectrums obtained in the step (5.1) to obtain the distance frequency spectrums after the 4 zero filling operations.
The specific method for zero filling operation of the 4 distance frequency spectrums comprises the following steps:
if the sub-band echo data corresponding to the 4 distance spectrums are subjected to distance spectrum expansion in the step two, the zero padding positions of the 4 distance spectrums are obtained according to the formula (1):
wherein fp isiAnd a zero padding position of the distance spectrum of the k-th strip of the ith sub-image from the off-line data.
If the sub-band echo data corresponding to the 4 distance spectrums are not subjected to distance spectrum expansion in the step two, the zero padding positions of the 4 distance spectrums are obtained according to a formula (2):
under the simulation parameters, the sub-band echo data corresponding to the 4 distance spectrums are subjected to distance spectrum expansion in the step two, so that the zero padding positions of the 4 distance spectrums are obtained by the formula (1) and are respectively 6.31MHz, 6.31MHz, 6.31MHz and 6.81 MHz.
Step 5.3: frequency shifting the 4 distance frequency spectrums after zero filling operation obtained in the step 5.2 to obtain 4 distance frequency spectrums after frequency shifting; symbol δ f for frequency shift amount of k-th line distance off-line data of ith sub-imageiIt is shown that,
step 5.4: and (4) superposing the 4 frequency-shifted distance spectrums obtained in the step (5.3), and performing inverse Fourier transform operation to finish distance spectrum reconstruction to obtain a high-resolution distance line.
In the finally obtained two-dimensional image with high distance resolution, a two-dimensional spectrogram of the point target is shown in fig. 2; the abscissa in fig. 2 is the azimuth frequency (sample point); the ordinate is the distance frequency (sample point); it can be seen from fig. 2 that the effect of subband data distance spectral aliasing is eliminated. Fig. 3 is a contour diagram of the target at this point, and the abscissa in fig. 3 is the azimuth direction (sampling point); the ordinate is the range direction (sample point); FIG. 4 is a graph of normalized magnitude of the target distance direction of the point, with the abscissa in FIG. 4 being the azimuth direction (sample point); the ordinate is the normalized amplitude; fig. 3 and 4 show that the point target is well focused. Fig. 2, 3 and 4 verify the correctness and effectiveness of the improved frequency-stepping SAR imaging algorithm based on frequency domain and frequency domain reconstruction.
The technical solutions of the present invention have been described with reference to specific embodiments, but these descriptions should not be construed as limiting the scope of the present invention, which is defined by the appended claims, and any modifications based on the claims are intended to be within the scope of the present invention.
Claims (5)
1. An improved frequency stepping SAR imaging method based on frequency domain spectrum reconstruction is characterized in that: the basic implementation process comprises the following steps of:
step one, grouping frequency stepping SAR echo data with n-point stepping carrier frequencies according to signal carrier frequencies to obtain n groups of sub-band echo data with different carrier frequencies, wherein n is more than or equal to 2 and n is a positive integer; each group of sub-band echo data comprises m pieces of distance off-line data, wherein m is a positive integer;
step two, judging whether the distance spectrum of the sub-band echo data in the step one is subjected to aliasing after two-dimensional focusing processing; if the frequency spectrum aliasing occurs, performing expansion operation on the distance frequency spectrum of the sub-band echo data, and then performing operation in the third step; otherwise, directly carrying out the operation of the third step;
step three, respectively carrying out SAR imaging operation on the n groups of sub-band echo data obtained in the step two to obtain n sub-images; when B is presentr>At Δ f, adjacent sub-band spectrums have an overlapping phenomenon, and in order to eliminate the influence of the overlapping of the sub-band spectrums on spectrum reconstruction, the overlapping part of the spectrums needs to be removed by windowing the distance spectrums during distance compression processing; wherein, BrIs a sub-band echo bandwidth and BrMore than or equal to delta f, wherein the delta f is a stepping frequency interval;
step four, carrying out azimuth registration on the n sub-images obtained in the step three to obtain n sub-images after azimuth registration;
step five, taking out kth distance offline data from each sub-image obtained in the step four to obtain n pieces of distance offline data, and performing distance spectrum reconstruction on the n pieces of distance offline data, wherein k is more than or equal to 1 and less than or equal to m, and k is a positive integer; and repeating the operation m times to obtain a two-dimensional image with high distance resolution.
2. The improved frequency-stepped SAR imaging method based on frequency-domain spectral reconstruction according to claim 1, wherein: the method for judging whether the distance spectrum of the subband echo data after the two-dimensional focusing processing is carried out in the second step is specifically as follows: if Δ f ≧ Δ fthresholdJudging that the distance spectrum of the sub-band echo data is subjected to aliasing after two-dimensional focusing processing; wherein, Δ fthresholdFor the purpose of the threshold step interval,
fsis the sampling rate; beta is aAIs the azimuth beam width; f. of0Is the carrier frequency; if Δ f<△fthresholdAnd judging that the distance spectrum of the sub-band echo data does not alias after the two-dimensional focusing processing is carried out on the sub-band echo data.
3. The improved frequency-stepped SAR imaging method based on frequency-domain spectral reconstruction according to claim 2, wherein: in the second step, the specific method for performing the operation of expanding the distance spectrum of the subband echo data comprises the following steps: converting sub-band echo data into a distance frequency domain through Fourier transform, and expanding the range of the distance frequency domain to B through zero filling at two ends of the distance frequency spectrumr_extendHz; wherein, alpha is an oversampling coefficient and 1<Alpha is less than or equal to 1.3; namely: if the original point number in the distance direction is marked with the symbol NrIndicating that the number of zero-filling points at both ends of the spectrum is denoted by the symbol Nr_zerosIt is shown that,indicating rounding up.
4. The improved frequency-stepped SAR imaging method based on frequency-domain spectral reconstruction according to claim 1, wherein: the SAR imaging operation in the third step comprises steps from 3.1 to 3.3, and specifically comprises the following steps:
step 3.1: performing distance compression operation on the sub-band echo data to obtain sub-band data subjected to distance compression operation; when B is presentr>At delta f, overlapping phenomena occur in adjacent sub-band frequency spectrums, and in order to eliminate the influence of the overlapping of the sub-band frequency spectrums on frequency spectrum reconstruction, overlapping parts of the frequency spectrums are removed through distance frequency spectrum windowing in distance compression processing;
step 3.2: performing range migration correction on the sub-band data subjected to the range compression operation to obtain sub-band data subjected to range migration correction;
step 3.3: and performing azimuth compression operation on the sub-band data after the range migration correction to obtain sub-images.
5. The improved frequency-stepped SAR imaging method based on frequency-domain spectral reconstruction according to claim 1, wherein: the specific method for carrying out azimuth registration on the n sub-images in the fourth step comprises the following steps: selecting the ith sub-image from the n sub-images as a reference image, wherein i is more than or equal to 1 and less than or equal to n and i is a positive integer, and translating the time shift property of other sub-images through Fourier transform in azimuthAnd each pixel is more than or equal to 1 and less than or equal to n, j is a positive integer and j is not equal to i.
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| CN102998655B (en) * | 2012-11-22 | 2014-07-30 | 北京理工大学 | Optimal wave position selecting method of synthetic aperture radar under scanning working mode |
| CN104133215B (en) * | 2014-05-29 | 2017-04-12 | 西安电子科技大学 | Synchronous orbit radar imaging method based on range migration fine adjustment and sub-band division |
| CN104181521B (en) * | 2014-06-24 | 2016-09-14 | 合肥工业大学 | A kind of high range resolution radar launching multifrequency carrier wave |
| CN104977571B (en) * | 2015-06-25 | 2017-06-20 | 西安电子科技大学 | Range ambiguity clutter suppression method based on pitching frequency diversity STAP |
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| CN111640147B (en) * | 2020-06-04 | 2023-10-31 | 北京无线电测量研究所 | SAR sub-image registration method, device and computer equipment for step frequency band stitching |
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